The invention relates to a golf club head. More particularly, the invention is related to a golf club head casting provided with an internal perimetral welt about the club face for differential cooling across the face of the club head during manufacture.
Metal woods have become extremely popular among golfers. Metal wood heads are fashioned from a variety of metals, including stainless steel, aluminum, and titanium, and are generally produced in investment casting operations in which molten metal is poured into a mold cavity and allowed to solidify.
Many solidification phenomena are known to occur during casting, and it is well known in the casting art that the structure and properties of a casting are directly affected by the shrinkage, cooling rate, and solidification time of the casting material.
Careful design of the mold and molding process allows the production of club heads with internal cavities and complex surfaces to be cast. Various methods and apparatus for producing golf club heads are disclosed in U.S. Pat. No. 4,842,243 to Butler, U.S. Pat. No. 5,056,705 to Wakita et al., U.S. Pat. No. 5,417,559 to Schmidt, U.S. Pat. No. 5,547,630 to Schmidt, and U.S. Pat. No. 5,651,409 to Sheehan, and are herein incorporated by reference.
In castings, defects can occur during the solidification process, especially due to solidification shrinkage as the casting cools. For example, when cooling from liquid to solid state, a low carbon steel typically shrinks 2.5%, while aluminum can shrink in excess of 6.5%. Additionally, as the solid-state casting cools to room temperature it may contract by several more percent. The shrinkage allowance for a steel casting can be one-quarter inch per foot of material.
In general, to account for shrinkage and avoid residual stresses, molds are often designed to limit the amount of restraint imposed on the casting during cooling. Since the casting of most materials will shrink during cooling, cracking and a concomitant low strength may result if the mold provides too much restriction on shrinkage and residual stresses are introduced. The mold should be compliant enough to permit the solidifying metal to contract in a predictable and desired manner. Thus, proper design of a mold and casting, and proper control of the casting process are essential to the production of club heads with consistent properties and dimensions.
Prior art metal club heads are generally produced from separate castings of a head-shell and soleplate. The club head is formed by welding the soleplate to the head-shell, and then finishing the surface of the head in a grinding and polishing operation.
A drawback of the casting process is that it is difficult to consistently cast the desired shape of the club head to a tight dimensional tolerance, accounting for the shrinkage that occurs during cooling. Yet, the proper shape and sizing of the club face is essential to achieving a desired performance in a golf club, especially in a metal wood.
The shape and sizing of a club face is quite complex. For example, the face progression, defined as the distance from the centerline of the shaft or hosel bore to the farthest front portion of the face on its centerline, is known to impact the trajectory of a golf ball. The face angle, defined as the angle of the face to the grounded sole line with the shaft hole perpendicular to the line of flight, impacts loft and direction of the golf ball, and thus the tendency of the ball to hook or slice.
Of great concern are two characteristics of the face, the horizontal face bulge and the vertical face roll. Horizontal face bulge radius is measured from the heel to toe or along the horizontal plane of the face, and is important because it compensates for a golfer""s hitting the ball off of the centerline. If a ball is hit at an off-center location, the bulge effectively compensates for the misalignment that would cause hooking or slicing. A typical wood has a horizontal face bulge radius of between 8 and 16 inches.
Vertical face roll radius is measured from the top of the face to the bottom of the face in a vertical position, and this factor affects the trajectory of the ball off the face. A typical wood has a vertical face roll radius of between 12 and 18 inches.
Prior art attempts to cast club heads to exacting specifications, as defined above, have met with poor results because of the thermomechanical behavior of the casting designs. For example, to minimize distortion of the casting during cooling, uniform cross-sections are desirable. Such a design criteria cannot be followed for a golf club head, especially the head of a wood containing an inner cavity. The face of the club head often contains grooving, and in the area near the hosel the cross section is often significantly wider than that of the remainder of the head-shell.
Prior art club head casting designs and casting methods often result in distortion of the shape and size of the clubhead during cooling. This is especially pronounced on the club head face, which although initially cast with an generally convex exterior surface, upon cooling often collapses inward and fails to retain the desired shape due to poor thermomechanical behavior during cooling.
Thus, there is a need for a casting that consistently results in the desired club head shape and sizing. More particularly, there is a need for a club head casting with a club head face having a consistent and predictable shape and size. Specifically, there is a need for a club head casting design and method which possess desirable thermomechanical behavior during cooling resulting in a casting with a face that does not excessively collapse upon cooling from casting temperature to room temperature.
The present invention relates to a golf club head casting adapted for attachment to a shaft. The golf club head includes a shell having an inner cavity with a head face and body, and a soleplate or crown plate coupled to the shell to form a substantially hollow body. The head face is cast and has an exterior surface, and the golf club head also includes means for thermomechanically stressing the head face during cooling, so that the exterior surface of the face is maintained in a generally convex shape.
Preferably, the golf club head face has a thickness of less than 0.1 inch. The convex shape may have a horizontal face bulge radius of less than 12 inches on at least a portion of the face, and may have a vertical face roll radius of less than 12 inches on at least a portion of the face The convex shape may alternatively have a vertical face roll radius of less than 8 inches on at least a portion of the face, and the head face and body may be cast simultaneously.
In another embodiment, a golf club head includes a homogeneous shell having an inner cavity with a head face and body, and a soleplate or crown plate coupled to the shell to form a substantially hollow body. The head face has an exterior surface and an interior surface, and a perimeter is included along the interior surface with a welt surrounding a substantial portion of the perimeter. Preferably, the welt covers more than 80 percent of the perimeter of the head face.
In a preferred embodiment, the welt covers more than 80 percent of the perimeter. The face may have a thickness between about 0.05 to 0.11 inch, and the welt may have a thickness between about 0.12 to 0.17 inch. In addition, a thickened central area of the head face may vary in thickness between about 0.050 and 0.150 inch.
In another embodiment, a golf club head includes a shell and a plate, the shell defining an inner cavity provided with a head face and a body. The body is provided with an aperture shaped for receiving the plate, and the head face is provided with a perimetral welt portion of a first thickness and a face portion of a second thickness adjacent the welt portion. The first thickness is at least 1.2 times greater than the second thickness. The perimetral welt is configured and dimensioned to cool slower than the central portion to maintain a predetermined face bulge and roll radius. Preferably, the head face thickness is less than 0.1 inch. The head face may have a generally convex shape with a horizontal face bulge radius of less than 12 inches on at least a portion of the face, and may have a vertical face roll radius of less than 12 inches on at least a portion of the face. The generally convex shape may alternatively have a vertical face roll radius of less than 8 inches on at least a portion of the face. The plate may be a soleplate or a crown plate.
The present invention also relates to a method of forming a close-tolerance casting of a golf club head shell. The method includes the steps of: selecting a face shape of a head shell corresponding to at least two face tolerance specifications, selecting a perimetral welt thickness and shape of a head shell to surround the face, forming a casting mold for casting the head shell having the face shape and perimetral welt, casting molten metal into the casting mold to form the head shell, and allowing the face to cool faster than the welt to maintain two face tolerance specifications. The at least two face tolerance specifications may be a horizontal face bulge radius and a vertical face roll radius.